Implantable medical devices such as pacemakers and implantable cardioverter defibrillators (ICDs) are important, life-saving technologies. When the body's own heart pacing mechanisms fail or degrade over time, life-threatening conditions such as arrhythmias and congestive heart failure may arise. Implantable medical devices monitor the electrical activity of the heart that occur with the heart contractions which pump blood. These devices further provide appropriate electrical stimulation when the heart's own pacing mechanisms function improperly or when the heart exhibits some form of arrhythmia such as cardioversion or defibrillation.
In general, implantable medical devices comprise a pulse or waveform generator, which generates a stimulating pulse or waveform, a plurality of electrodes positioned proximate the body tissue which transmit the stimulation pulses or waveforms to the body tissue, and an insulated electrical conductor which interconnects the pulse or waveform generator and the electrode. The electrode and electrical conductor are referred to as a lead. Generally, at least one electrode is positioned at a distal portion of the lead. Leads may be placed in a variety of locations on a patient's heart, depending on their individual heart condition, and allow the implantable medical devices to sense the heart's electrical activity and provide appropriate stimulation when necessary.
A common method of positioning heart stimulation leads in contact with the patient's heart is through transvenous insertion. In this process, an incision is made in a vein near the location of the implanted stimulation device casing and the heart stimulation lead is advanced through the large veins that lead to the heart. An x-ray tube, or fluoroscope, is often used to visualize the leads and heart structures so that the leads can be placed in a satisfactory position.
The composition of the lead body is subject to stringent performance requirements for both implantation and long-term service. In one aspect, it is desirable that the lead body be chemically biocompatible with body media it is exposed to in service, so as not to cause harm to the body. In another aspect, it is desirable that the lead body exhibit selected mechanical properties. For example, the distal portion of the lead should possess sufficient flexibility so as to navigate the tortuous pathways of the veins leading to the heart. Concurrently, the proximal portion of the lead should possess sufficient stiffness so as to facilitate pushing the lead through the vein, as well as sufficient wear resistance to withstand long term use within the heart, without significant erosion or scratching damage. If pronounced, these forms of damage may result in mechanical failure of the lead body.
One approach which has been developed to meet these varying performance requirements has been through the use of composite leads. Composites are material systems which combine two or more distinct materials or phases, each with its own distinctive, desirable properties, to create a new material with desirable properties that may not be present, or to the same extent, in the components alone. For example, composite lead bodies having a distal portion formed from silicone, for flexibility, and a proximal portion, formed from silicone-polyurethane co-polymers for stiffness and wear resistance, have been demonstrated.
While these composite leads possess the chemical and mechanical properties necessary for long term use within their respective portions, the junction at which the materials are joined is problematic. In some composite lead systems, medical adhesives or transition joints are employed to attach the proximal and distal portion materials together. Traditional medical adhesives can progressively weaken over time, however, reducing the bond strength between the materials and raise the risk of lead body separation and exposure of the electrical conductor to the body media. Furthermore, mechanical couplings, such as transition joints, may locally increase the stiffness and/or cross-sectional diameter of the lead about the junction to a degree where it is difficult or impossible to properly navigate the leads through the veins.
These deficiencies in the design of conventional heart stimulation leads illustrate the need for improved heart stimulation leads which impart desired chemical and mechanical characteristics to the lead body, while maintaining the ease of implantation.